• The Hydrozoa include solitary or colonial cnidarians, which have a noncellular mesoglea, lack tentacles within the gastrovascular cavity, and have no gullet.• It comprises 3 major orders containing fewer than 3,000 species which are mosly marine.
• Two common hydroids in rockpools around Sydney. Both are members of the Leptomedusae with the polyps protected by hydrothecal cups. The triangular structures in right colony are the cups protecting individual polyps. Size: Left - each erect frond aprox 10mm. Right - each branch approx 5mm.
• Marine hydroids usually exist as colonies, the polyps are protected by a horny perisarc which.• The colony can either be a branching sessile structure attached to the substrate or it can have erect fern-like fronds with microscopic polyps arranged along the individual branches.
There are two major suborders of hydroids:S.O. Anthomedusae. The polyp is not protected by the exoskeleton which stops at the base of the polyp. (Also known as gymnoblastic hydroids or Athecata)S.O. Leptomedusae. Both the polyp and specialised gonad structures are protected by exoskeleton cups. (Also known as Calyptoblastic hydroids, Thecata).
• All hydroids are carnivorous animals, catching prey in the water column with the aid of stinging and grappling nematocysts.• Aeolids and some dendronotaceans, such as Doto, prey on most families of hydroids, and in most cases each nudibranch species specialises on one or a few species of hydroid.
Order Siphonophora• These are the pelagic colonies of hydrozoa which either drift on the ocean surface or pulsate through the midwater.• The best known is Physalia, the Portugese man-o-war. A modified free-floating polyp develops into a large gas-filled float from which other polyp and medusiod individuals are produced by budding.
• These threads of tiny hydroids can reach lengths of more then 30 meters!• Each individual is approximately 1 cm.Physalia, the Portugese man-o- war
The Hydrocorals• Are colonial hydrozoans that have a hard calcified supporting skeleton.• The millepore hydrocorals are commonly known as fire or stinging corals (they can inflict a serious sting).• Hydrocorals are essentially warm water animalsOrder: MilleporinaStylasterina.
Anatomy of a polyp. These animals have analmost plant-like appearance, being anchored inplace.
• Anthozoans are exclusively marine, polypoid cnidarians.• the largest class of cnidarians, containing over 6,000 species.• A gullet extends for a short distance into the gastrovascular cavity, and septa are present, which increase the surface for digestion and absorption.• Anthozoans are colonial or solitary organisms.
Subclass Alcyonaria• includes almost universally colonial organisms in which each of the polyps, or hydroid members, has eight feathery tentacles.• Most of them produce a skeleton, and many make some contributions to coral reefs.• While some are found in temperate seas, they are especially common in subtropical to tropical regions.
• The organ pipe coral (Tubipora), a soft coral (Alcyonium), the Indo-Pacific blue coral (Heliopora), and the sea pens, which have a stalk extending into the bottom mud or sand, are some typical alcyonarian corals. Horny corals, of the order Gorgonacea, are perhaps the best known.
Subclass Zoantharia• includes both solitary and colonial forms, in which the polyp has more than eight tentacles.• The solitary sea anemones belong here, in the order Actiniaria, characterized by the lack of a skeleton.
• The stony corals so important in forming coral reefs belong to the order Madreporaria; they are especially characterized by their calcium carbonate exoskeleton, marked by many cups for the polyps, each of which contains stony septa dividing the gastrovascular cavity into compartments.
• The shape of coral skeletons depends on the pattern of growth of the colony. Ufelina